Conventionally, 590 MPa grade steels have been used for automotive chassis and impact members such as bumpers and center pillars because they demand formability (mainly ductility and stretch flangeability). Recently, however, the use of automotive steel sheets with higher strengths has been promoted to reduce the effects of automobiles on the environment and to improve crashworthiness, and research on the use of 980 MPa grade steels has been started. In general, a steel sheet having a higher strength has a lower workability. Therefore, steel sheets having high strength and high workability have been currently researched. Examples of techniques for improving ductility and stretch flangeability include the following techniques.
Japanese Unexamined Patent Application Publication No. 2007-063668 discloses a technique related to a high-tensile-strength steel sheet having a tensile strength of 980 MPa or more, the steel sheet being composed substantially of a ferritic single-phase structure and having carbides of an average grain size of less than 10 nm precipitated and dispersed therein, the carbides containing titanium, molybdenum, and vanadium and having an average composition satisfying V/(Ti+Mo+V)≧0.3, where Ti, Mo, and V are expressed in atomic percent.
Japanese Unexamined Patent Application Publication No. 2006-161112 discloses a technique related to a high-strength hot-rolled steel sheet having a strength of 880 MPa or more and a yield ratio of 0.80 or more, the steel sheet having a steel composition containing, by mass, 0.08% to 0.20% of carbon, 0.001% to less than 0.2% of silicon, more than 1.0% to 3.0% of manganese, 0.001% to 0.5% of aluminum, more than 0.1% to 0.5% of vanadium, 0.05% to less than 0.2% of titanium, and 0.005% to 0.5% of niobium and satisfying inequalities (a), (b), and (c), the balance being iron and impurities, and a steel structure containing 70% by volume or more of ferrite having an average grain size of 5 μm or less and a hardness of 250 Hv or more:9(Ti/48+Nb/93)×C/12≦4.5×10−5  Inequality (a):0.5%≦(V/51+Ti/48+Nb/93)/(C/12)≦1.5  Inequality (b):V+Ti×2+Nb×1.4+C×2+Mn×0.1≧0.80.  Inequality (c):
Japanese Unexamined Patent Application Publication No. 2004-143518 discloses a technique related to a hot-rolled steel sheet containing, in mass percent, 0.05% to 0.2% of carbon, 0.001% to 3.0% of silicon, 0.5% to 3.0% of manganese, 0.001% to 0.2% of phosphorus, 0.001% to 3% of aluminum, more than 0.1% to 1.5% of vanadium, and optionally 0.05% to 1.0% of molybdenum, the balance being iron and impurities, the steel sheet having a structure containing ferrite having an average grain size of 1 to 5 μm as a primary phase, the ferrite grains containing vanadium carbonitrides having an average grain size of 50 nm or less.
Japanese Unexamined Patent Application Publication No. 2004-360046 discloses a technique related to a high-strength steel sheet having a tensile strength of 880 MPa or more in a direction perpendicular to a rolling direction and a yield ratio of 0.8 or more, the steel sheet having a steel composition containing, in mass percent, 0.04% to 0.17% of carbon, 1.1% or less of silicon, 1.6% to 2.6% of manganese, 0.05% or less of phosphorus, 0.02% or less of sulfur, 0.001% to 0.05% of aluminum, 0.02% or less of nitrogen, 0.11% to 0.3% of vanadium, and 0.07% to 0.25% of titanium, the balance being iron and incidental impurities.
Japanese Unexamined Patent Application Publication No. 2005-002406 discloses a technique related to a high-strength hot-rolled steel sheet having a strength of 880 MPa or more and a yield ratio of 0.80 or more, the steel sheet having a steel composition containing, in mass percent, 0.04% to 0.20% of carbon, 0.001% to 1.1% of silicon, more than 0.8% of manganese, 0.05% to less than 0.15% of titanium, and 0% to 0.05% of niobium and satisfying inequalities (d), (e), and (f), the balance being iron and incidental impurities:(Ti/48+Nb/93)×C/12≦3.5×10−5  Inequality (d):0.4≦(V/51+Ti/48+Nb/93)/(C/12)≦2.0  Inequality (e):V+Ti×2+Nb×1.4+C×2+Si×0.2+Mn×0.1≧0.7.  Inequality (f):
Japanese Unexamined Patent Application Publication No. 2005-232567 discloses a technique related to an ultrahigh-tensile-strength steel sheet with excellent stretch flangeability having a tensile strength of 950 MPa or more, the steel sheet being composed substantially of a ferritic single-phase structure, the ferritic structure having precipitates containing titanium, molybdenum, and carbon precipitated therein, wherein the area fraction of <110> colonies of adjacent crystal grains in a region between a position one-fourth of the thickness and a position three-fourths of the thickness in a cross section perpendicular to a vector parallel to a rolling direction is 50% or less.
Japanese Unexamined Patent Application Publication No. 2006-183138 discloses a technique related to a steel sheet having a composition containing, in mass percent, 0.10% to 0.25% of carbon, 1.5% or less of silicon, 1.0% to 3.0% of manganese, 0.10% or less of phosphorus, 0.005% or less of sulfur, 0.01% to 0.5% of aluminum, 0.010% or less of nitrogen, and 0.10% to 1.0% of vanadium and satisfying (10Mn+V)/C≧50, the balance being iron and incidental impurities, wherein the average grain size of carbides containing vanadium determined for precipitates having a grain size of 80 nm or less is 30 nm or less.
Japanese Unexamined Patent Application Publication No. 2006-183139 discloses a technique related to an automotive member having a composition containing, in mass percent, 0.10% to 0.25% of carbon, 1.5% or less of silicon, 1.0% to 3.0% of manganese, 0.10% or less of phosphorus, 0.005% or less of sulfur, 0.01% to 0.5% of aluminum, 0.010% or less of nitrogen, and 0.10% to 1.0% of vanadium and satisfying (10 Mn+V)/C≧50, the balance being iron and incidental impurities, wherein the volume fraction of tempered martensite phase is 80% or more, and the average grain size of carbides containing vanadium and having a grain size of 20 nm or less is 10 nm or less.
Japanese Unexamined Patent Application Publication No. 2007-016319 discloses a technique related to high-tensile-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer thereon, the steel sheet having a chemical composition containing, in mass percent, more than 0.02% to 0.2% of carbon, 0.01% to 2.0% of silicon, 0.1% to 3.0% of manganese, 0.003% to 0.10% of phosphorus, 0.020% or less of sulfur, 0.001% to 1.0% of aluminum, 0.0004% to 0.015% of nitrogen, and 0.03% to 0.2% of titanium, the balance being iron and impurities, the steel sheet having a metallographic structure containing 30% to 95% by area of ferrite, wherein if second phases in the balance include martensite, bainite, pearlite, and cementite, the area fraction of martensite is 0% to 50%, the steel sheet containing titanium-based carbonitride precipitates having a grain size of 2 to 30 nm with an average intergrain distance of 30 to 300 nm and crystallized TiN having a grain size of 3 μm or more with an average intergrain distance of 50 to 500 μm.
Japanese Unexamined Patent Application Publication No. 2003-105444 discloses a technique related to a method for improving the fatigue resistance of a steel sheet, including subjecting a steel sheet to strain aging treatment to form fine precipitates having a grain size of 10 nm or less, the steel sheet having a composition containing, in mass percent, 0.01% to 0.15% of carbon, 2.0% or less of silicon, 0.5% to 3.0% of manganese, 0.1% or less of phosphorus, 0.02% or less of sulfur, 0.1% or less of aluminum, 0.02% or less of nitrogen, and 0.5% to 3.0% of copper and having a multiphase structure containing ferrite phase as a primary phase and a phase containing 2% by area or more of martensite phase as a second phase.
Japanese Unexamined Patent Application Publication No. 4-289120 discloses a technique related to a method for manufacturing an ultrahigh-strength cold-rolled steel sheet with good formability and strip shape having a fine two-phase structure containing 80% to 97% by volume of martensite, the balance being ferrite, and a tensile strength of 150 to 200 kgf/mm2, the method including hot-rolling a steel at a finishing temperature higher than or equal to the Ar3 point, coiling the steel at 500° C. to 650° C., pickling the steel, cold-rolling the steel, performing continuous annealing by heating the steel to Ac3 to [Ac3+70° C.] and soaking the steel for 30 seconds or more, performing first cooling to precipitate 3% to 20% by volume of ferrite, quenching the steel to room temperature in a jet of water, and subjecting the steel to overaging treatment at 120° C. to 300° C. for 1 to 15 minutes, the steel containing, in mass percent, 0.18% to 0.3% of carbon, 1.2% or less of silicon, 1% to 2.5% of manganese, 0.02% or less of phosphorus, 0.003% or less of sulfur, and 0.01% to 0.1% of dissolved aluminum and further containing one or more of 0.005% to 0.030% of niobium, 0.01% to 0.10% of vanadium, and 0.01% to 0.10% of titanium in a total amount of 0.005% to 0.10%, the balance being iron and incidental impurities.
Japanese Unexamined Patent Application Publication No. 2003-096543 discloses a technique related to a high-strength hot-rolled steel sheet having high bake hardenability at high prestrain, the steel sheet containing, in mass percent, 0.0005% to 0.3% of carbon, 0.001% to 3.0% of silicon, 0.01% to 3.0% of manganese, 0.0001% to 0.3% of aluminum, 0.0001% to 0.1% of sulfur, and 0.0010% to 0.05% of nitrogen, the balance being iron and incidental impurities, wherein ferrite has the largest area fraction, dissolved carbon, Sol. C, and dissolved nitrogen, Sol. N, satisfy Sol.C/Sol.N=0.1 to 100, and the average or each of the amounts of increase in yield strength and tensile strength after prestraining to 5% to 20% and baking at 110° C. to 200° C. for 1 to 60 minutes is 50 MPa or more as compared to the steel sheet before prestraining and baking.
However, the known techniques described above have the following problems.
The steels described in JP '668, JP '112 and JP '518, which contain molybdenum, noticeably increase cost because the price of molybdenum has been rising recently. In addition, steel sheets for automotive applications have been used in severely corrosive environments in foreign countries as the automotive industry has globalized, which demands higher corrosion resistance after coating of steel sheets. The addition of molybdenum, however, cannot meet the above demand because it impairs formation or growth of conversion crystals, thus decreasing the corrosion resistance after coating of the steel sheets. Therefore, the steels described in JP '668, JP '112 and JP '518 do not satisfactorily meet the recent demand in the automotive industry.
On the other hand, a working process including, in sequence, drawing or stretch forming, piercing, and flange forming has been employed with the recent advances in pressing technology. This working process requires the portion of a steel sheet subjected to stretch flanging to have stretch flangeability after drawing or stretch forming and piercing, that is, after working. The steels described in JP '668, JP '112, JP '518, JP '046, JP '406, JP '567, JP '138, JP '139, JP '319, JP '444, JP '120 and JP '543, however, do not necessarily have sufficient stretch flangeability after working because this property has only recently been noted.
Among the common techniques for strengthening steel is precipitation strengthening. It is known that the amount of precipitation strengthening is inversely proportional to the grain size of precipitates and is proportional to the square root of the amount of precipitate. For example, the steels disclosed in JP '668, JP '112, JP '518, JP '046, JP '406, JP '567, JP '138, JP '139, JP '319, JP '444, JP '120 and JP '543 contain carbonitride-forming elements such as titanium, vanadium, and niobium; particularly, JP '138, JP '319 and JP '444 have conducted research on the size of precipitates. However, the amount of precipitate is not necessarily sufficient. A high cost due to low precipitation efficiency is problematic.
Niobium, added in JP '112, JP '406 and JP '120, significantly inhibits recrystallization of austenite after hot rolling. This causes a problem in that it leaves unrecrystallized grains in the steel, thus decreasing workability, and also causes a problem in that the rolling load in hot rolling is increased.
In light of the above circumstances, it could be helpful to provide a high-strength steel sheet having excellent stretch flangeability after working and a method for manufacturing such a steel sheet.
As a result of our study in providing a high-strength steel sheet having excellent stretch flangeability after working and a tensile strength of 980 MPa or more, we discovered the following findings:                (i) To provide a high-strength steel sheet, it is necessary to form fine precipitates (less than 20 nm in size) and to increase the proportion of fine precipitates (less than 20 nm in size). Fine precipitates that can be maintained include those containing titanium-molybdenum or titanium-vanadium. In view of alloy cost, composite precipitation of titanium and vanadium is useful.        (ii) Stretch flangeability after working improves if the difference in hardness between the ferrite phase and a second phase is −300 to 300. In addition, a structure having excellent stretch flangeability after working can be formed by controlling first cooling stop temperature T1 and coiling temperature T2 to the respective optimal ranges.        
We thus provide:                [1] A high-strength steel sheet having a composition containing, in mass percent, 0.08% to 0.20% of carbon, 0.2% to 1.0% of silicon, 0.5% to 2.5% of manganese, 0.04% or less of phosphorus, 0.005% or less of sulfur, 0.05% or less of aluminum, 0.07% to 0.20% of titanium, and 0.20% to 0.80% of vanadium, the balance being iron and incidental impurities, the steel sheet having a metallographic structure including 80% to 98% by volume of a ferrite phase and a second phase, wherein the sum of the amounts of titanium and vanadium contained in precipitates having a size of less than 20 nm is 0.150% by mass or more, and the difference (HVα−HVS) between the hardness (HVα) of the ferrite phase and the hardness (HVS) of the second phase is −300 to 300.        [2] The high-strength steel sheet in [1] above, wherein the amount of titanium contained in precipitates having a size of less than 20 nm is 0.150% by mass or more.        [3] The high-strength steel sheet in [1] above, wherein the amount of vanadium contained in precipitates having a size of less than 20 nm is 0.550% by mass or more.        [4] The high-strength steel sheet in one of [1] to [3] above, further containing, in mass percent, one or more of 0.01% to 1.0% of chromium, 0.005% to 1.0% of tungsten, and 0.0005% to 0.05% of zirconium.        [5] A method for manufacturing a high-strength steel sheet, including heating to a temperature of 1,150° C. to 1,350° C. a steel slab having a composition containing, in mass percent, 0.08% to 0.20% of carbon, 0.2% to 1.0% of silicon, 0.5% to 2.5% of manganese, 0.04% or less of phosphorus, 0.005% or less of sulfur, 0.05% or less of aluminum, 0.07% to 0.20% of titanium, and 0.20% to 0.80% of vanadium, the balance being iron and incidental impurities, hot-rolling the steel slab at a finish rolling temperature of 850° C. to 1,000° C., subjecting the hot-rolled steel sheet to first cooling to a temperature of 650° C. to lower than 800° C. at an average cooling rate of 30° C./s or higher, cooling the steel sheet with air for one to less than five seconds, subjecting the steel sheet to second cooling at a cooling rate of 20° C./s or higher, and coiling the steel sheet at a temperature of higher than 200° C. to 550° C., wherein inequality (1) is satisfied:T1≦0.06×T2+764  inequality (1)        wherein T1 is first cooling stop temperature (° C.) and T2 is coiling temperature (° C.).        [6] The method for manufacturing a high-strength steel sheet in [5] above, wherein the composition further contains, in mass percent, one or more of 0.01% to 1.0% of chromium, 0.005% to 1.0% of tungsten, and 0.0005% to 0.05% of zirconium.        
The percentages used herein for steel compositions are all expressed by mass. In addition, the term “high-strength steel sheet” as used herein refers to a steel sheet having a tensile strength (hereinafter also referred to as “TS”) of 980 MPa or more and includes hot-rolled steel sheets and those subjected to surface treatment such as plating, that is, surface-treated steel sheets.
In addition, we achieve stretch flangeability (λ10) of 40% or more after rolling to an elongation of 10%.
Accordingly, a high-strength steel sheet having excellent stretch flangeability after working and a TS of 980 MPa or more can be provided. Our steel sheets and methods allow for cost reduction because the above advantages are achieved without adding molybdenum. When used for applications such as automotive chassis, frames for trucks, and impact members, our high-strength steel sheet allows a reduction in thickness, thus reducing the effects of automobiles on the environment, and significantly improves crashworthiness.